In recent years, a material for forming particularly a color image is predominating as an image recording material. Specifically, an inkjet recording material, a heat-sensitive transfer recording material, an electrophotographic recording material, a transfer silver halide light-sensitive material, a printing ink, a recording pen and the like are popularly used. Also, a color filter for recording and reproducing a color image is used, in the case of the filming equipment, in an imaging device such as CCD and, in the case of the display, in LCD or PDP. In these color image recording materials and color filters, colorants (dyes or pigments) of three primary colors by a so-called additive or subtractive color mixing method are used for displaying or recording a full color image, but a colorant having absorption characteristics capable of realizing a preferred color reproduction region and having fastness high enough to endure various use conditions and environmental conditions is not found at present, and improvements are keenly demanded.
The dyes and pigments used in the above-described applications are commonly required to have the following properties. For example, it is required to have good absorption characteristics in terms of color reproduction and show good fastness to usage environment conditions, such as light fastness, heat resistance and resistance to an oxidative gas such as ozone. In addition, in the case where the colorant is a pigment, the requisite properties further include, for example, being substantially insoluble in water or an organic solvent, showing good chemical resistance, and not impairing the preferred absorption characteristics in the molecular dispersion state even when used as a particle. These requisite characteristics can be controlled by the degree of the intermolecular interaction, but absorption characteristics and fastness are in a trade-off relationship and therefore, it is difficult to satisfy both at the same time.
Moreover, in using a pigment, other than the properties described above, it is also required, for example, to have a particle size and a particle shape necessary for bringing out the desired transparency, to show good fastness to usage environment conditions, such as light fastness, heat resistance, resistance to an oxidative gas (e.g., ozone), and chemical resistance to an organic solvent, a sulfurous acid gas or the like, and to be capable of dispersing even into a microparticle in the medium used and keeping stable the dispersed state. Above all, a pigment having good hue and being fast to light, wet heat and environmental active gases, in particular, a pigment having high tinctorial strength and being fast to light, is strongly demanded.
More specifically, the performance required of the pigment is diversified as compared with the dye that is required to have performances as a colorant molecule, and not only performances as a colorant molecule but also all of the above-described requisite performances as a solid (fine particle dispersion) resulting from aggregation of colorant molecules must be satisfied. In turn, the compound group usable as a pigment is extremely limited as compared with the dye and even when a pigment is derived from a high-performance dye, the pigment capable of satisfying the requisite performances as a fine particle dispersion is very few in number and cannot be easily developed. This can be confirmed also by the fact that the number of pigments registered in the Color Index is less than 1/10 of the number of dyes.
Azo pigments are widely used in a printing ink, an inkjet ink, an electrophotographic material and the like because of their excellent coloristic characteristics, i.e., hue and tinctorial strength. Of these azo dyes, a yellow diarylide pigment and a red naphthol azo pigment are most typically used. Examples of the diarylide pigment include C.I. Pigment Yellow 12, C.I. Pigment Yellow 13 and C.I. Pigment Yellow 17, and examples of the naphthol azo pigment include C.I. Pigment Red 208 and C.I. Pigment Red 242. However, these pigments are very poor in the fastness, particularly light fastness, and when the printed material is exposed to light, the pigment is decomposed to cause fading. Thus, these are not suitable for storage of the printed material for a long period of time.
In order to overcome such a defect, an azo pigment improved in the fastness by increasing the molecular weight or introducing a group having strong intermolecular interaction is disclosed (see, for example, Patent Documents 1 to 3). However, even the improved pigment is still insufficient, though the light fastness of the pigment described, for example, in Patent Document 1 is improved. Also, the pigments described, for example, in Patent Documents 2 to 3 bring about green tinting in the hue and decrease in the tinctorial strength and disadvantageously suffer from poor coloristic characteristics.
In Patent Documents 4 and 5, colorants having excellent absorption characteristics for color reproduction and sufficiently high fastness are disclosed. However, all of specific compounds described in these patent documents dissolve in water or an organic solvent and are insufficient in the chemical resistance.
In the case of producing a full color by a subtractive color mixing method using three colors of yellow, magenta and cyan or four colors with the addition of black, when a pigment poor in the fastness is used as the pigment for one color, the gray balance of the printed material is changed with the passage of time, and when a pigment poor in the coloristic characteristics is used, the color reproducibility at the printing is decreased. Accordingly, for obtaining a printed material capable of maintaining high color reproducibility for a long period of time, a pigment or pigment dispersion satisfying both coloristic characteristics and fastness is demanded.
Many of azo colorants have various visible light absorptions and therefore, have been conventionally utilized as a colorant in various fields. The azo colorant is being used in various fields, for example, for coloring a synthetic resin, as a colorant for printing ink or sublimation-type heat-sensitive transfer material, or as a colorant for inkjet ink or color filter. The major performance as a colorant, which is required of an azo colorant, includes an absorption spectrum. The hue of a colorant greatly affects the color tone, touch and the like of an article colored with the colorant and gives a great effect on the visual perception. Therefore, studies have been long made on the absorption spectrum of colorant.
A conventionally known azo dye using a nitrogen-containing 5-membered ring as the azo component is disclosed also in Patent Documents 6 and 7.
On the other hand, many of representative organic pigments have polymorphs, and such a pigment is known to take two or more crystal morphologies, despite the same chemical composition.
Some organic pigments can be obtained, like an azo pigment, as fine and size-regulated particles by selecting appropriate reaction conditions at the synthesis, some can be obtained, like a copper phthalocyanine green pigment, as a pigment after very fine aggregated particles produced at the synthesis are subjected to particle growth and size regulation in the post-process, and some can be obtained, like a copper phthalocyanine blue pigment, as a pigment after coarse irregular particles produced at the synthesis are subjected to pulverization and size regulation in the post-process. For example, a diketopyrrolopyrrole pigment is generally synthesized by reacting a succinic acid diester and an aromatic nitrile in an organic solvent (see, for example, Patent Document 8). The crude diketopyrrolopyrrole pigment is heat-treated in water or an organic solvent and then subjected to disintegration such as wet grinding, whereby the pigment is obtained in the form suitable for use (see, for example, Patent Document 9). For example, C.I. Pigment Red 254 that is a diketopyrrolopyrrole pigment is known to have α-type and β-type crystal morphologies (see, for example, Patent Document 10). Also, C.I. Pigment Yellow 181 that is an azo pigment is known to have several kinds of crystal morphologies (see, for example, Patent Document 11).
Patent Document 1: JP-A-56-38354 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 2: U.S. Pat. No. 2,936,306
Patent Document 3: JP-A-11-100519
Patent Document 4: JP-A-2005-213357
Patent Document 5: JP-A-2003-246942
Patent Document 6: JP-A-55-161856
Patent Document 7: JP-A-2002-371214
Patent Document 8: JP-A-58-210084
Patent Document 9: JP-A-5-222314
Patent Document 10: JP-A-8-48908
Patent Document 11: U.S. Patent Application Publication 2008/058531, specification